1. Field of the Invention
The invention concerns a method and device for protection against a jammer in a radio station having several change-over antennas and their use for the making of radiogoniometers.
2. Description of the Prior Art
To ensure the minimum quality of communications in radio-electrical transmission systems or, again, for reasons related to manufacturing costs, it is common practice to use only a limited number of reception channels, which are smaller than the number of antennas belonging to the station, by changing over the reception channels to the most appropriate antennas, depending on propagation circumstances or, again, on requirements for processing the received data. This is especially the case for diversified radioelectrical transmission systems and for radiogoniometers working in a sequential mode. In the former case, for example, the changing over is done on the antenna most suited to the tropospheric or ionospheric propagation conditions in the latter case, to limit manufacturing costs, the processing unit responsible for computing the angle of arrival of the electromagnetic waves performs antenna change-over operations to undertake interferometrical measurements on a limited number of reception channels.
The counterpart of this is that these change-over operations introduce spectral aliasing due to the convolution of the spectrum of the signals present at the antenna before filtering by the reception channel and the convolution of the spectrum of the change-over signal. This is all the more troublesome as, when there is a jammer, i.e. a transmitter having its frequency locked into frequency close to the station's operating frequency, a part of the spectrum resulting from the convolution of the change-over signal with the signal of the jammer penetrates the reception frequency band. To the extent that the power of the jammer is far greater than that of the useful signal, that portion of the power of the jamming signal which is aliased in the useful band may be greater than the power of the signal. In these circumstances, the processing of the received signals becomes disturbed or perhaps even impossible.